Recently, a phenomenon has been reported that magnetic domain walls in a magnetic nanowire are moved by a current flowing through the magnetic nanowire. Attempts have been made to constitute shift registers using this phenomenon. The magnetic nanowires used in the attempts have, at their central portions, recording elements for writing magnetic domain walls in the magnetic nanowires, and reproducing elements for detecting the directions of the magnetization in magnetic domains. Electrodes for causing a current to flow through each magnetic nanowire are provided at both terminals of the magnetic nanowires. The magnetic domain walls are shifted by passing a current through the corresponding magnetic nanowire. The data is recorded as the magnetization direction of the magnetic domain sectioned by the magnetic domain wall, as in the case of other magnetic recording apparatuses such as hard disks. A shift register is proposed in which a magnetic domain containing data is shifted by a current so that the target data can be accessed, and the target data can be read by detecting the magnetization direction of the magnetic domain by a reproducing element disposed at a central portion of the magnetic nanowire.
If a plurality of aforementioned shift registers are integrated to constitute a large-capacity memory, the magnetic nanowires preferably extend in the vertical direction to reduce the area per bit. Also from the viewpoint of disposing electrodes at both ends of the magnetic nanowires and forming corresponding wiring lines on a wafer substrate, the magnetic nanowires preferably extend in a direction perpendicular to the wafer substrate.
The recording element and the reproducing element (hereinafter also referred to as the “recording and reproducing element”) disposed at the central portion of the magnetic nanowire are also preferably formed on the wafer substrate since they are formed through a semiconductor lithography process.
In order to obtain a structure in which a recording and reproducing element is formed on a wafer substrate so as to be in contact with a central portion of a magnetic nanowire extending in a direction perpendicular to the wafer substrate, a method is proposed in which the magnetic nanowire is bent to have a U shape so that the bent portion is in contact with the recording and reproducing element on the wafer substrate. In this case, the structure includes electrodes on a plane for causing a current to flow to move magnetic domain walls, and a plurality of recording and reproducing elements on an opposite plane, and magnetic nanowires connecting the electrodes and the recording and reproducing elements, the magnetic nanowires being bent to have a U shape. Both terminals of each U-shaped magnetic nanowire are in contact with the electrodes, the bent portion is in contact with one of the recording and reproducing element, and straight portion is perpendicular to the wafer substrate.
A large-capacity shift register type magnetic memory is thus obtained by three-dimensionally setting magnetic nanowires in a direction perpendicular to a substrate to reduce the area per bit to be recorded, and by shaping the magnetic nanowires to have a U-shape so that the central portion of each magnetic nanowire contacts a recording and reproducing element formed on the wafer substrate.
As described above, the large-capacity shift register type magnetic memory enables each recording and reproducing element on the wafer substrate to be in contact with the central portion of the corresponding magnetic nanowire by three-dimensionally arranging the magnetic nanowire so as to be perpendicular to the wafer substrate to reduce the area per bit to be recorded, and by shaping the magnetic nanowire to have a U shape.
The diameter and the pitch of the magnetic nanowire to obtain a large-scale memory with a capacity of a few T bits or more per one chip (100 mm2), for example, are on the order of a few ten nm, and the length is on the order of μm. A method proposed to form such a magnetic nanowire arrangement includes forming perpendicular holes by, for example, deep dry etching by means of a Bosch process, and filling the holes with a magnetic material. However, it is difficult to form fine holes with the aspect ratio of a few hundred or more with the Bosch process. Furthermore, it is difficult to process U-shaped holes.
Anodizing is another method proposed to form a magnetic nanowire arrangement with a high aspect ratio. In this method, a voltage is applied to an anode formed of an aluminum substrate in an electrolytic solution so that the dissolution of aluminum and the growth of a resultant element are advanced by oxidation to form holes in aluminum oxide. Holes with the pitch of a few ten nm and the depth of a few hundred μm can be formed by controlling the voltage and the solution. Magnetic nanowires meeting the target recording density can be obtained by filling the holes with a magnetic material. However, this method cannot form a U-shaped structure easily. Furthermore, since the anodizing advances somewhat irregularly, it is difficult to ensure a complete control of magnetic nanowire arrangement and a linear formation of magnetic nanowires with this method.
A method of producing a large-scale shift register type magnetic memory is being studied, in which a wafer substrate on which an electrode for a current to drive magnetic domain walls is formed, a wafer substrate on which recording and reproducing elements are formed, and a substrate on which perpendicular magnetic nanowires are formed by the aforementioned method are separately prepared, and the three substrates are bonded together with the alignment among them being precisely made. The substrate on which the electrode for moving magnetic domain walls is formed and the substrate on which the recording and reproducing elements are formed are bonded from opposite sides to the substrate in which the magnetic nanowires are formed. In bonding the substrates, the substrates should be positioned correctly and accurately. However, since the pitch of the magnetic nanowires is a few ten nm, it is difficult to bond the substrate correctly and accurately with the present techniques. In some cases, the correct positioning among the patterns on all of the three substrates is impossible. For example, anodizing may not always form holes regularly and linearly in the substrate in which magnetic nanowires are to be formed. Therefore, regardless of the positioning accuracy, the electrode for moving the magnetic domain walls and the recording and reproducing elements may not be connected with the magnetic nanowires so that each magnetic nanowire corresponds to one of the recording and reproducing elements only by the positioning of the three substrates.
There is another problem in that the magnetic domain walls may stop moving at the bent portion of the U-shaped magnetic nanowire. If this occurs, the shift operation may not be performed even if a current flows.